Tom Arnold -- I'm NOT Voting For Roseanne

It's still too early to call ... but it looks like Roseanne will NOT be the next President of the United States ... and her ex-husband Tom Arnold is partly to blame.

Of course, Roseanne claims to be a serious candidate to be the next leader of the free world ... running as the nominee of the Peace and Freedom Party.

In case you haven't followed her campaign, here's a summary -- she hates both Democrats and Republicans and REAAALLLLLY wants to legalize marijuana. That's pretty much it.

But her ex-hubby Tom Arnold tells TMZ ... she just hasn't earned his vote -- saying, "I'm voting for Obama, but for God's sake let Roseanne have her pot ... she's 60."

REMINDER -- Don't forget to vote today ... or Sophia Bush will be ashamed of you.

6:30 AM PT -- Roseanne isn't getting much help from her old castmates either.